Weird Shrink/Fatten Scalling Problem
As Akados mentioned, there is a problem with your normals.
What are normals?
Every face has a normal. It's the direction the face is pointing at. One side of every triangle is considered the &quot;outer&quot; side and the other is the &quot;inner&quot; side.
Why is shrink/fatten confused?
When you use shrink/fatten, it moves the faces in the direction of the normals.
Fatten = move towards the &quot;outer&quot; side.
Shrink = move towards the &quot;inner&quot; side.
It makes sense when you think about it.
Since the &quot;3&quot; in your example has the correct normals, it works fine for that part of the mesh. But the &quot;D&quot; part seems to have flipped normals, so the shrink/fatten actually moves faces in the opposite direction.
How to fix it?
Blender knows how to automatically get the normals right. You have to select everything in the mesh and Make normals consistent, using the shortcut ControlN.